Various processes have been known and utilized in the prior art for reducing the temperature difference across a reboiler-condenser such as providing the maximum possible heat transfer surface area and/or by enhancing the heat transfer coefficient of the boiling and/or condensing fluid. Generally, in the heat transfer equipment used previously, two heat transfer process schemes have been employed. Both of these process arrangements have the condensing vapor entering at the top of the heat exchanger with the condensate flowing downwards under gravity to exit at the bottom.
One arrangement of the boiling process, termed downflow boiling, is to introduce the liquid at the top of the heat exchanger and allow it to boil while draining under gravity. This has the benefit of a small pressure change with height since the adverse effect of liquid head is largely eliminated. Thus, the boiling temperature of the liquid remains approximately constant along with the temperature difference between boiling and condensing fluids; this helps to maximize the efficiency of the reboiler-condenser. This arrangement has been used infrequently because of the difficulty of distributing liquid uniformly and the necessity to provide an external liquid pumping system to achieve sufficient liquid flow to ensure that the boiling liquid flows over the whole of the heat transfer surface. ln an air separation plant, this is necessary for safety reasons as well as to maintain a high heat transfer performance of the boiling surface.
The more common heat transfer process places the heat exchanger in a bath of the boiling liquid so that the boiling surface is immersed. Vapor formed at the boiling surface rises due to buoyancy and carries liquid with it. This induces an upward circulating liquid flow through the boiling zone, with fresh liquid being drawn into the bottom of the zone and excess liquid being discharged at the top end and hence being recirculated to the bottom inlet. This process is termed thermosyphon boiling.
Various types of equipment are known for these above boiling processes. The earliest form was the shell and tube reboiler with boiling either inside or outside of the tubes and using either downflow or thermosyphon schemes. ln one improvement the area for heat transfer was increased for the thermosyphon process, and thus the temperature difference reduced, by the introduction of the brazed aluminum reboiler.
ln a typical heat exchanger of this design, aluminum plates, designated as parting sheets, 0.03 to 0.05 inches thick are connected by a corrugated aluminum sheet which serves to form a series of fins perpendicular to the parting sheets. Typically the fin sheets will have a thickness of 0.008 to 0.012 inches with 15 to 25 fins per inch and a fin height, the distance between parting sheets, of 0.2 to 0.3 inches. A heat exchanger is formed by brazing an assembly of these plates with the edges enclosed by side bars.
This exchanger is immersed in a bath of the liquid to be boiled with the parting sheets and the fins orientated vertically, Alternate passages separated by the parting sheets contain the boiling and condensing fluids. The liquid to be boiled enters the open bottom of the boiling passages and flows upward under thermosyphon action. The resulting heated mixture of liquid and vapor exits via the open top of the boiling passages. The vapor to be condensed is introduced at the top of the condensing passages through a manifold welded to the side of the heat exchanger and having openings into alternate passages. The resulting condensate leaves the lower end of the condensing passages through a similar side manifold. Special distributor fins, inclined at an angle to the vertical, are used at the inlet and outlet of the condensing passages. The upper and lower horizontal ends of the condensing passages are sealed with end bars.
Attempts to increase the effectiveness of both types of heat exchangers operating by the thermosyphon process have also been made by enhancement of the heat transfer coefficient. ln the shell-and-tube heat exchanger, nucleate boiling promoters have been used consisting of a porous metal layer approximately 0.010 inch thick which is bonded metallurgically to the inner tube surface. Heat transfer coefficients in nucleate boiling are enhanced 10-15 fold over a corresponding bare surface. A combination of extended microsurface area and large numbers of stable re-entrant nucleation sites are responsible for the improved performance. The external tube surface is also enhanced for condensation by the provision of flutes on the surface.
Enhanced boiling heat transfer surface has also been applied to the brazed aluminum heat exchanger by scribing the primary boiling surface with many fine lines to promote nucleation. At the same time the boiling passage fins were eliminated. This type of reboiler is described in U.S. Pat. No. 3,457,990 of N. P. Theophilos and D. I-J. Wang.
In both of these types of enhanced reboiler-condensers a single type of heat transfer surface is used throughout the vertical height of the boiling circuit and thus the essentially uniform pressure gradient and varying temperature distribution of the single zone thermosyphon process is preserved with its attendant inefficiency.